1. Field of the Invention
The present invention relates to an image sensing apparatus and a control method for the image sensing apparatus.
2. Description of the Related Art
In image sensing apparatuses, such as digital cameras, image sensors, such as charged-coupled device (CCD) sensors, are often used. In the image sensors, dark current components may be contained in image signals output from a pixel array, in which a plurality of pixels are arranged. If image signals output from pixels include a great amount of dark current components, the quality of an image formed by the image signals deteriorates.
In the pixel array of the image sensor, any pixel that outputs a signal which includes a considerable amount of dark current components is referred to as a defective pixel. Even if defective pixels occur at such a level that several pixels are defective in the entire pixel array of the image sensor, the quality of an image obtained by signals from the pixel array is noticeably impaired. It is considered that defective pixels occur during the manufacture of an image sensor, or after manufacturing, due to radiation of high-energy electromagnetic waves, such as cosmic rays. It is also considered that the defective pixels that have been detected once can be constantly detected at least under the same temperature conditions.
As a countermeasure for defective pixels, a process described below is considered as effective. Specifically, image signals for one frame are output from the pixel array under the same predetermined conditions (same temperature and same accumulation time under the light shielding condition), and a pixel that corresponds to a signal of a predetermined level or higher is detected as a defective pixel from the image signals for the one frame. Then, the signal from the detected defective pixel is interpolated using signals from the surrounding pixels.
Meanwhile, there are also defective pixels at which the signal level of the read signal is either high at one time or low at another time if the signal is read a number of times under the same conditions (same temperature and same accumulation time under the light shielding condition) as when the pixel was detected as a defective pixel. Those defective pixels behave the way they had flicker defects, that is, they act as normal pixels at one time and defective pixels at another time. With regard to this problem, Japanese Patent Application Laid-Open No. 2003-37781 discusses an image sensor in which a pixel containing a flicker defect can be detected using image signals of a plurality of frames.
However, with a method discussed in Japanese Patent Application Laid-Open No. 2003-37781, because image signals of a plurality of frames are acquired, it is necessary to perform a lengthy process of detecting a pixel having a flicker defect and interpolating a signal of the defective pixel (a defective pixel adjusting process). Consequently, the takt time in a camera manufacturing process (the defective pixel adjusting process) may be prolonged, thus increasing production cost.